"Today, we're happy to make available a developer preview of the Native Client SDK - an important first step in making Native Client more accessible as a tool for developing real web applications. When we released the research version of Native Client a year ago, we offered a snapshot of our source tree that developers could download and tinker with, but the download was big and cumbersome to use. The Native Client SDK preview, in contrast, includes just the basics you need to get started writing an app in minutes."

Not only native client is sandboxed, what you compile
runs on any platform , even your cellphone (given you compile an ARM target together with the x86 one). ActiveX used the windows API, this uses a portable, HTML5-friendly api. In the future, pNaCl will add an extra layer to ensure that you compile it one and runs on any platform/